About The Cover

A vapor deposit of ice warmed to 183 kelvin, much as cometary ice is heated during transit through the solar system, in a false-color transmission electron microscope image (×l70,000). On warming, initially well-defined crystallites flow into a rolling landscape (blue). Diffraction studies reveal both amorphous and cubic crystalline components. These persist until at a higher temperature all ice transforms into the familiar hexagonal form. See page 753. [Micrograph: P. Jenniskens and D. F. Blake]